1. Field of the Invention
This invention relates to an ink jet recording method and an ink jet recording apparatus in which ink is discharged to thereby accomplish recording, and in particular, to an ink jet recording method and an ink jet recording apparatus in which pressure is imparted to ink in a liquid path by vibration of an electromechanical conversion element and ink droplets are discharged from an orifice communicating with said liquid path.
2. Related Background Art
FIG. 1 of the accompanying drawings is a schematic front view showing an example of an ink jet printer constructed by the use of an ink jet recording head.
In FIG. 1, a platen 7 is rotatively driven in conformity with the printing situation by a line feed motor 2 which is a pulse motor, and conveys recording paper, not shown, in the circumferential direction thereof. The presence of this recording paper is detected by a paper sensor 4. An ink jet recording head (hereinafter simply referred to as the recording head) 3 is constructed of a plurality of liquid paths 30 communicating with discharge ports for discharging ink therethrough to effect ink injection, and is provided on a carriage slidably provided on a guide bar, not shown, and is moved along the platen 7 through a belt 6 by the drive of a carriage motor 5 comprising a DC motor. A linear encoder 8 and an encoder sensor 9 are provided to effect the detection of the position of the recording head 3 in that case, and a home position sensor 10 is provided to effect the detection of the home position. Also, an ink sucking cap 12 movable by the drive of an auto cap motor 11 is provided to recover the liquid paths 30 of the recording head 3 from their unsatisfactory ink droplet injection, and the operative position of the cap 12 is detected by a cap sensor 13.
Control of the ink jet printer of such construction is effected by a conventional CPU (central processing unit) 20 in the construction of a control system shown in FIG. 2 of the accompanying drawings.
A group of switches 21 and a group of sensors 25 and in addition, the encoder sensor 9 and the home position sensor 10 are connected to the input portion of the CPU 20, and a DC servo reversing circuit 22 for driving the carriage motor 5, a pulse motor driving circuit 23 for driving the line feed motor 2 and a head driver 24 for driving the recording head 3 are connected to [he output portion of the CPU 20. Also, a memory 26 storing record data therein is connected to the CPU 20 through a bus line.
The CPU 20 executes the following control in response to an operation input effected by the group of switches 21, for example, switches or the like provided on an operating panel (not shown). That is, the CPU 20 refers to the input from the encoder sensor 9 and the home position sensor 10, controls the drive of the carriage motor 5 through the DC servo reversing circuit 22, controls the drive of the line feed motor 2 through the pulse motor driving circuit 23, outputs the record data D from the memory 26 to the head driver 24 as pulse generating means, and drives the recording head 3 by the head driver 24. The CPU 20 also effects the control of other mechanisms, not shown, in response to the input from another group of sensors 25 such as ambient temperature sensors.
Under such a construction, the recording operation is started by a print switch in the group of switches 21 being closed. The presence of recording paper is first confirmed by the paper sensor 4, whereafter the line feed motor 2 is driven by several steps to rotate the platen 7 and the recording paper is set at the recording start position. Subsequently, the carriage motor 5 is driven to reciprocally move the recording head 3, and in synchronism therewith, the line feed motor 2 is driven to feed the recording paper line by line. In the meantime, a drive signal conforming to the record data is applied from the head driver 24 to the recording head 3, whereby the recording head 3 is driven and ink droplets are injected from the liquid paths 30 and thus, characters or images are recorded.
FIG. 3 of the accompanying drawings is a schematic cross-sectional view showing the structure of the liquid path 30 of the recording head 3.
The fore end portion of a liquid path body 31 forming a pipe-like shape and into which the ink is introduced is formed into a conical shape and is provided with a discharge port 31a as an opening for discharging ink droplets therethrough. The surface of the ink 35 which is exposed to the outside through this discharge port 31a becomes concave meniscus M.
Also, a cylindrical piezo-electric element 32 which is an electro-mechanical conversion element for producing discharge pressure for ink droplets and which has electrodes 36 and 37 on the inner and outer peripheral surfaces thereof is fitted to the outer surface of the central portion of the liquid, path body 31, and is adhesively secured to the outer surface in a completely integral state with respect to the liquid path body without leaving any gap therebetween, by an adhesive agent such as epoxy resin.
Further, a filter 34 is fixedly fitted to the vicinity of the open rear end of the liquid path body 31. The filter 34 has the functions of cutting off dust and like foreign materials in the ink 35 flowing to the left as indicated by the arrow from the rear end of the liquid path body 31 and supplied to the orifice leading to the liquid paths, and taking the acoustic impedance to a sound wave produced in the liquid path body 31 during the driving of the piezo-electric element 32 which will be described hereinbelow, at the fore end side and the rear end side of the liquid path.
The piezo-electric constant of such a piezoelectric element 32 is, e.g. about -300.times.10 m/v with respect to a certain direction and about 600.times.10 m/v with respect to another direction, and thus, a very high piezo-electric property is exhibited.
FIG. 4 of the accompanying drawings shows the wave forms of driving voltages applied to the piezo-electric element 32 by the head driver 24. A driving voltage V1 in the opposite direction to the polarization of the piezo-electric element 32 is imparted, whereby the piezo-electric element 32 is expanded and the amount of ink in the liquid path increases (before the application of the voltage, the liquid path is filled with ink through the filter 34). When a driving voltage V2 is then imparted in the direction of polarization, the piezo-electric element 32 contracts, whereby a pressure wave is produced. This pressure wave propagates through the ink 35 at a speed of about 1,200 m/s-1,500 m/s. By the combined effect of the contraction of the piezo-electric element 32 and the propagation of the pressure wave, an ink droplet is discharged from the discharge port 31a. In FIG. 4, P indicates the direction of polarization and P.sub.O indicates the direction opposite to the polarization. This also holds true in FIGS. 6 and 10 of the accompanying drawings.
FIG. 5 of the accompanying drawings shows a variation in the amount of retraction of meniscus resulting from vibration of the piezo-electric element. The amount of retraction M.sub.R of meniscus becomes greatest after the discharge of an ink droplet, whereafter it becomes a vibration attenuated by the influences of the force of restitution and the pressure wave produced by the aforedescribed contraction of the piezo-electric element 32.
However, when the peak of the vibration of meniscus is great, there has been the undesirable possibility of the ink leading from the discharge port 31a at the depending portion of time t1 and conversely, there has been the undesirable possibility of a bubble entering into the liquid path at the intermediate residing portion of time t2. Accordingly, it has been difficult in some cases to obtain images of stable and good quality by the use of the conventional driving wave form shown in FIG. 4.
To eliminate such an inconvenience, U.S. Pat. No. 4,112,433 (Vernon) and U.S. Pat. No. 4,101,646 (Fischbeck) have been proposed. These constructions are such that a main pulse is first applied and then a sub-pulse is applied in a predetermined time. An attempt has been made to attenuate the amplitude of the vibration of meniscus by such constructions to thereby prevent bubbles from entering into the liquid path as much as possible.
Also, U.S. Pat. No. 4,409,596 (Ishii) has proposed such a construction that a driving signal comprising two pulses is applied to the recording head. One of the two pulses is a main pulse for discharging ink which is generated with a predetermined delay with respect to a timing pulse synchronized with the movement of the carriage, and the other pulse is a pulse for not discharging ink which is generated before the main pulse is generated, and the timing thereof can be shifted to vary the interval thereof with respect to the main pulse. The initial driven state of the meniscus in the liquid path is dynamically stabilized by the first pulse, and an ink droplet is stably injected by the subsequent main pulse. At this time, by shifting the timing of the first pulse, the state of the meniscus during the application of the main pulse can be varied, whereby the volume of ink can be varied with a result that harmony could be changed.
However, in the conventional ink jet recording apparatus, vibration of meniscus could not be appropriately suppressed simply by applying a predetermined sub-pulse.
That is, the viscosity of the ink varies with a variation in temperature and therefore, even if there is no variation in the driving voltage, meniscus varies in conformity with temperature. Accordingly, the volume of the ink droplet discharged varies, and this has led to the problem that stable recording cannot be accomplished.
Also, there has occured the problem that it is difficult to correct the timing of the first pulse in conformity with a variation in the volume of ink and the circuit therefor is complex in construction. Further, when the main pulse is to be generated with a predetermined delay from a predetermined timing pulse synchronized with the movement of the carriage, there has occurred the inconvenience that when the recording head is reciprocally driven, there must be provided a circuit for correcting the point at which an ink droplet is shot on a recording medium.
In addition, the amplitude of the meniscus vibration after the discharge of an ink droplet is varied by temperature and therefore, depending on the temperature condition, leakage of excess ink or entry of bubbles into the liquid path has occurred and the time required for the meniscus to restore its steady state has become long in some cases.